This invention was made with government support under Contract No. N00019-02-C 3003 awarded by the United States Air Force. The government therefore has certain rights in this invention.
This application relates to a method of predicting flutter of a rotor using an aeroelastic model based on principal shapes of modes (AMPS).
Most systems encountered from day to day are stable, however, a rotor in a gas turbine engine can be aero-mechanically unstable under certain conditions. During engine operation, the rotors spin rapidly and the air flows between the rotor blades at high speeds. Under these circumstances, the aerodynamics can be such as to destabilize a structural mode of the rotor, i.e. when the unstable mode is briefly excited, for example, by just a small gust of air, the amplitude of the vibratory response will grow instead of diminish. This phenomenon is known as “flutter.”
Flutter, within turbo-machinery, is said to occur when a structure, such as a rotor blade or the rotor itself, begins to vibrate in the absence of any external forcing. The vibration due to flutter can result in a catastrophic engine failure. Specifically, flutter may cause a blade or blades to eventually crack and break resulting in damage to the engine. Therefore, when flutter is detected within the expected operating range of the engine, redesign is usually required.
In the interest of efficiency and performance, modern aircraft designers are tasked with designing an aircraft that is lighter. This generally entails more flexible structures, and more flexible structures bring an increased risk of flutter. The engine design cycle that involves designing, building, testing and redesigning is known as “build-and-bust” and is a very expensive way to develop an aircraft engine. As such, if the engine could be initially designed without flutter, it would result in an enormous cost savings to the engine maker.
As is known, while flutter is still commonly detected in initial engine designs, over the years, design tools including numerical methods have improved flutter prediction, reducing the number of build-and-bust cycles required to certify an engine. Known numerical methods include reduced order models, which are fast and computationally inexpensive, however their range of applicability is limited. Conversely, a higher order approach, such as a direct method, while applicable to a broader range of problems, is computationally expensive, as the direct method involves simulating each mode of interest.
These known direct method approaches of flutter prediction require running a great number of very large computational fluid dynamics (CFD) calculations, which can take many days or weeks to complete. As such, it is desirable to provide a method of flutter prediction that reduces both the number and the size of calculations required.